There are many systems, particularly those used in aerospace fluid power applications, such as propellant feed systems, wherein the operating cycles, pressure supplies, or fluid volumes are either fixed or otherwise limited. This makes it mandatory that the pressure drop caused by a filter be controlled in terms of degree and rate of increase in order to prevent flow decay and to maintain proper system pressure, mixture ratios and response times.
This degree and rate at which the differential pressure of a filter of a specific filtration efficiency increases during a typical operational cycle of a given system, however, is affected by a number of constantly changing conditions. These conditions are the amount of entrained contaminants, the rate of contaminant generation, the size of the particles generated, and the physical and chemical composition of the contaminants. In addition, the differential pressure buildup of the filter varies as a function of a number of other conditions or combinations thereof. These functions are the type of system, the type of fluid medium, the flow rate, system pressure, total fluid flow, pressure and flow surges, vibration and other environmental conditions during the complete operational cycle.
Even when a filter is properly sized, however, the present methods or configurations provide no means of determining how much of the useful service life of the filter has been expended at any point during its period of operation, nor do they provide any warning of the approaching end of the service life cycle. This is due to the fact that the initial pressure loss caused by the filter at rated flow is almost entirely due to its port connections and other restrictions within the filter housing. Thus, the pressure drop of the filter does not increase significantly until most of its service life has been expended. As soon as the increase does become measurable, however, the pressure drop rises asymptotically until it reaches the systems operating pressure. This high pressure, in turn, makes it necessary to provide supporting structures on the downstream side of the filtering element to prevent its collapse at the maximum transient pressures of the system. The supporting structure further increases the size and weight of the filter assembly.
In addition, the uncontrolled buildup of differential pressure across the filter element causes trapped particles to be forced deeper into the filter medium capillaries and makes it difficult to reclean such units after usage. It is common practice to clean or replace the filter element at periodic overhaul periods and as a result, most filters in use are replaced or recleaned far too frequently which adds unnecessary operational costs.
Another problem area associated with the present methods and configurations is caused by the fact that as the filter removes contaminants from the fluid, the mean pore size of the filtering medium gets smaller due to partial or complete blockage of the capillaries. This results in the retention of contaminants from the fluid which are smaller than the filter is initially designed to remove to protect the system. This retention of smaller particles, in turn, accelerates the rate of differential pressure buildup towards the end of the filter's service life.